1. Field of the Invention
The present invention relates to a fixing device including a nip pressure applying mechanism. In addition, the present invention also relates to an image forming apparatus, such as a copier, a printer, and a facsimile machine, including the fixing device.
2. Description of the Background
In electrophotographic image forming apparatuses such as copiers and laser printers, a toner image is formed on an image bearing member (e.g., a photoconductor) and transferred onto paper. Finally, the toner image is fixed on the paper by a fixing device.
Fixing devices are of two main types: heat-roller-type fixing devices and belt-type fixing devices. In the heat-roller-type fixing device, a fixing roller equipped with a heat source and a pressing roller are in contact with each other at a predetermined pressure. (The portion where the pressing roller is pressed against the fixing roller may be hereinafter referred to as the “nip portion”.) In the belt-type fixing device, a fixing belt is stretched across multiple rollers including a fixing roller and a heating roller, and a pressing roller is pressed against the fixing roller at a predetermined pressure with the fixing belt therebetween, thus forming the nip portion. In both types of fixing devices, a paper having a toner image thereon passes through the nip portion and receives heat and pressure therefrom. Thus, the toner image is fixed on the paper.
Both the fixing roller and the pressing roller generally have an elastic layer on their surfaces. When such fixing roller and pressing roller are kept in a static contact state at a specific nip pressure for an extended period of time, the elastic layers may permanently deform and never return to the original state even when the nip pressure is released. Such a nip portion formed with the deformed elastic layers generates noise during fixing operation and produces low-quality images.
Electrophotographic image forming apparatuses have another problem of blistering including paper blistering and toner blistering.
Paper blistering generally occurs when a toner image is fixed on a sheet of paper by application of heat. Upon heating of the paper, moisture inside the paper generates moisture vapor, and the moisture vapor gradually increase its pressure. When the moisture vapor cannot be smoothly discharged from the paper due to the presence of a surface coating layer on the paper, the moisture vapor rapidly expands, causing local blistering within the paper.
Toner blistering generally occurs when a toner image is fixed on paper, especially a coated paper or a paper already having a fixed toner image on the opposite side, by application of heat. Air and moisture existing between toner particles or between toner particles and the paper expand upon heating. Because the expanded air and moisture cannot easily penetrate such coated paper or paper having a fixed toner image on the opposite side, bubbles are generated within the toner image.
In attempting to solve the problem of blistering, Japanese Patent Application Publication No. 2005-215580 proposes a mechanism for separating a pressing roller from a fixing member for a predetermined distance while image formation is not occurring so that the pressing roller will not be heated above a predetermined temperature.
However, disadvantageously, this mechanism takes an unnecessarily long time to bring the pressing roller into contact with the fixing member again to be ready for fixing a next toner image.